In the planar technology of semiconductor devices and microcircuits widely known is a method of drying a protective polymer coating applied onto a surface of an article from a solution, including a stage of holding the coating at temperatures of the process room, a stage of subsequent holding at an elevated temperature under an excessive pressure, and a stage of cooling. The method is intended to tackle the problems of producing high-quality defect-free polymer films and maintaining their protective properties during a certain period of time. The protective properties of coatings are greatly affected by microcracks, blisters, which are formed and propagated depending on physico-chemical processes, occurring during drying (as used herein drying is a process of escape of a solvent from the polymer coating).
Drying-out of the coating material (drying) includes the processes of transfer of the solvent in the polymer itself (liquid diffusion), in gaseous atmosphere (gas diffusion) and a transition of the solvent from the liquid state into vapor (a phase transformation of the first kind). Kinetic characteristics of the above-indicated processes determine the mechanism of escape of the solvent from the coating.
The initial stage of drying, which is carried out at temperatures of the process room (20.degree.-24.degree. C.), is characterized by a high content of the solvent in the polymer coating and high rates of its liquid diffusion. As the coating becomes dry, the solvent diffusion in the polymer material is retarded, which leads to lowering the drying rate.
It is known that by holding the polymer coating only at temperatures of the process room, it is impossible to obtain high values of protective properties (adhesion, defect-free condition).
Therefore, the temperature is held at the second stage at an elevated temperature. The temperature is selected for reasons of necessity to obtain a high flow of the polymer for increasing the rate of diffusion and relieving internal stresses of the coating, but with reference to thermostability of the polymer material, since a high temperature and its long action result in undesirable reactions of thermodestruction or thermopolymerization.
At the initial stage of holding of the polymer coating at an elevated temperature there registers a sharp increase of the rate of escape of the solvent. Having then reached its maximum value, this rate drops to zero.
It is thought that in parallel with evaporation of the solvent, on the external surface of the coating there occurs a phase transition (evaporation) also on the surface of gas microcracks, present in the coating (internal vapor formation).
Experiments confirm a connection between internal vapor formation and protective properties of, for example, a photoresistive coating. Kinetic characteristics of the process of evaporation of the solvent inside a microcrack are determined by a great number of factors: content of the solvent in the coating, pressure of the solvent vapor, initial forms and sizes of microcavities, density of distribution of microcracks in the coating volume, coefficient of liquid diffusion of the solvent, toughness and surface tension of the polymer coating, and by temperature of the coating.
The increase of the rate of internal vapor formation is accompanied by increase of unsoundness and decrease of adhesion of the coating to an article. The phenomena occur as a result of propagation of microcracks, saturated with vapors of the solvent, and subsequent opening of them on the coating external surface.
The location of closed microcracks relative to the coating external surface and the coating-to-article boundaries exert a great effect on the process of defects formation. Thus, microcracks located close to the external surface of the coating, when opening, do not affect deep layers of the polymer coating and do not exert such effect on the protective properties and adhesion, which is exerted by microcracks located on the article-to-polymer coating boundary. The latter, in addition to separation of the coating, When opening on the external surface, create through punctures. Microcracks located in the middle layers of the coating create prerequisites for the formation of punctures and, as areas of concentration of internal stresses of the coating, weaken its adhesion to the article..
Internal vapor generation in the protective coating with a high content of the solvent at the first stage of drying does not exert a noticeable effect, which decreases the protective properties of the polymer coating.
This is explained by the following reasons: favorable conditions for evaporation of the solvent, associated with a high coefficient of liquid diffusion in the coating and a state of a high flow of the polymer solution, which is greatly conducive to healing of developing microcracks.
Decreasing of the solvent content is accompanied by an increase of viscosity of the polymer coating and a loss of the planarization ability (as used herein the planarization ability is an ability of the polymer to create a flat surface and to heal irregularities of edges of the opened microcracks).
It is impossible to accurately determine the moment, when the solution flow will be insufficient for healing microcracks of dangerous forms and sizes, inasmuch as there are no relevant methodics. It is only possible with a certain truth, confirmed by practical data, to think that for the majority of photoresistive coatings this happens upon completion of centrifuging, at which excess of the polymer protective coating are removed.
Considering gas microcracks as a cause of originating punctures in the protective coating and as a cause of weakening the adhesion, it is necessary to point out to a polydispersive character of their sizes and forms. Naturally, with the diminishing of large microcracks, the protective properties of coatings are particularly improved.
At the same time, duration of storage of dried coatings may render dangerous even small microcracks.
Probably, for each process, depending on the conditions of its carrying out and requirements to protective properties, the maximum permissible size of microcracks may be determined at which deterioration of protective properties below the necessary values does not take place.
The nearest to the invention is a method of drying the protective polymer coating, applied onto an article from the solution, including a stage of holding the polymer coating within the range of temperatures of the process room in the time interval from 20 s to 1 hour, a stage of subsequent holding of the polymer coating at am elevated temperature under excessive pressure, sufficient for suppressing the process of propagation of microcracks in the coating, deteriorating its protective properties, and a stage of cooling of the polymer coating (V. P. Lavrischev, V. A. Peremyschev "Study of mechanism of removing the solvent from the photoresist film", 1975 Electronics, issue 5 (53), pages 58-65).
The factor preventing the action of internal vapor formation and suppressing the propagation of gas microcracks is an excessive external pressure, when holding the protective coating under conditions of an elevated temperature.
The polymer coating dried according to the above-indicated method has defects in the form of punctures and weak places. The method features loss of adhesion, which shortens the working life of the polymer coating. Both drawbacks are initiated by the process of propagation of microcracks of the coating during its drying.
Suppression of this process at the stage of holding the polymer coating at an elevated temperature does not always ensure fulfilment of technical requirements to protective properties of polymer coatings.
Known in the art is a device for effecting a method of drying the protective polymer coating applied onto an article from the solution, including a high-temperature air-tight chamber for holding the polymer coating at an elevated temperature, connected to a high-pressure main and an open outlet into the external volume through pipelines with valves, having a loading hatch with an air-tight damper, an unloading hatch with an air-tight damper, a mechanism for moving the article from the loading hatch to the unloading hatch, and a heater ("Electronic industry" No. 5 (77), 1979, Moscow, V. V. Anufrienko, V. I. Osnin, V. A. Peremyschev, V. L. Sanderov, V. N. Tsarev "Unit for forming photoresist coatings AFF-2", pages 50-52).
In such devices the excessive pressure is built up at the stage of holding the polymer coating at elevated temperatures. In this case, building-up of the excessive pressure in a high-temperature chamber is possible only after loading the article inside the chamber and closing the loading hatch with am air-tight damper, whereas releasing of this pressure is effected prior to the moment when the coating is cooled to the temperature at which, under conditions of the normal pressure, eliminated is the process of propagation of microcracks in the coating, deteriorating protective properties of the coating.
Even an inconsiderable delay of the excessive pressure action relative to heating of the coating has a noticeable negative effect owing to a small (fractures of a micrometer) thickness and heat capacity of the coating. After bringing the coating into the heated volume of the high-temperature chamber, it gets heated practically instantaneously to the temperature of the gas in the chamber. But it is impossible to build up excessive pressure instantaneously. In such heated state the coating is at the moment of releasing pressure prior to opening of the high-temperature chamber for unloading the article. But heating of the polymer coating without the excessive pressure brings about intensification of the internal vapor generation, which forms favorable conditions for propagation of microcracks in the protective coating.